We correlate electron transport data directly with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the ${\text{VO}}_2$ metal-insulator transition. A set of vanadium oxide thin films were prepared by reactive dc sputtering from a V target at various oxygen partial pressures (${\text{O}}_2$ p.p.). Metal-insulator transition (MIT) characteristic to ${\text{VO}}_2$ can be seen from the temperature dependence of electrical resistance of the films sputtered at optimal ${\text{O}}_2$ p.p. Lower and higher ${\text{O}}_2$ p.p. result in disappearance of the MIT. The results of the near edge x-ray absorption fine structure spectroscopy of the $\text{O}K$ edge in identical VO films are presented. Redistribution of the spectral weight from ${\sigma }^{\ast }$ to ${\pi }^{\ast }$ bands is found in the vanadium oxide films exhibiting stronger ${\text{VO}}_2$ MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We also observe a clear correlation between MIT and the width and area of the lower ${\pi }^{\ast }$ band, which is likely to be due to the emergence of the $d_{\left|\right|}$ band overlapping with ${\pi }^{\ast }$. The strengthening of this $d_{\left|\right|}$ band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the $d_{\left|\right|}$ band and electron correlations in the phase transition.

• Received 4 December 2007

DOI:https://doi.org/10.1103/PhysRevB.77.195442